The present invention relates to an antenna and feeder cable tester, and more particularly to the testing of antennas and feeder cables associated with a base station in a mobile telephone communication system.
Mobile communications systems, such as cellular telephone systems, typically rely on the use of radio for communicating with mobile subscribers. As illustrated in FIG. 1, such systems have a fixed site, such as a cellular telephone system base station (BS) 101, coupled to a transmitting (TX) antenna 105 by means of a TX feeder cable 103. The BS 101 is further coupled to a receiving (RX) antenna 109 by means of an RX feeder cable 107. When the system is operational, radio frequency signals are transmitted from the TX antenna 105 to a mobile subscriber 111. Signals transmitted by the mobile subscriber 111 are received by the RX antenna 109 and supplied to the BS 101 by means of the RX feeder cable 107.
Because proper functioning of the TX and RX feeder cables 103, 107 and TX and RX antennas 105, 109 is critical to the communications system, these are tested at the time of installation, and continue to be monitored during operation. Two methods of monitoring the operation of the TX and RX feeder cables 103, 107 and TX and RX antennas 105, 109 are conventionally used: direct measurement of the voltage standing wave ratio (VSWR) and the use of statistical methods. A testing apparatus 113 is provided in the BS 101 for performing either or both of these tests.
For testing the transmission path, a direct VSWR measurement may be performed by measuring the VSWR of the feeder cable 103 and/or the TX antenna 105 using the power of a signal that has already been transmitted by the BS 101. Using the testing apparatus 113 situated in the BS 101, power leaving the BS 101 is compared with the power reflected into the BS 101 from the TX feeder 103. A low return loss indicates a good TX feeder/TX antenna combination (everything has been radiated); a high return loss indicates that something is wrong (e.g., broken TX feeder 103, missing TX antenna 105, etc.). The measurement function is realized with a directional coupler and power detectors, which are well-known in the art. The power detectors may alternatively be of the narrowband or wideband variety. In time division multiple access (TDMA) systems, the power detectors may work on a per timeslot basis (comparing forward and reverse power in each timeslot). In non-TDMA systems, average power may be detected. The test of the TX antenna 105 may further include checking the condition of the transmitters (not shown) by measuring the forward power.
In order to test the receiving path, a VSWR measurement of the feeder cable 103 and/or the antenna 105 may be made by injecting a test tone signal into the base station side of the RX feeder cable 107 and measuring the reflected signal. Alternatively, receiving path testing may merely involve statistical methods, such as correlating received signal strength with a known distance between the BS 101 and the mobile subscriber 111. (Distance between the BS 101 and the mobile subscriber 111 may be determined at the BS 101 by measuring the duration of time from the transmission of a burst from the BS 101 until a reply is received from the mobile subscriber 111. In time division multiple access (TDMA) systems, the "turnaround" time of the mobile subscriber 111 is adjustable in accordance with a command from the BS 101, in order to enable the slots from different mobile subscribers to arrive at one base station receiver in good TDMA order without overlapping.) A signal strength lower than expected might be indicative of a problem with either or both of the RX antenna 109 and the RX feeder cable 107.
In their simplest form, statistical methods may also be used to check the overall health of the communications system, including the TX and RX antennas 105, 109 and feeder cables 103, 107. That is, if communications traffic is exchanged, then the system is deemed to be functioning properly. If traffic hasn't been exchanged for a certain amount of time, then a problem is detected.
The above-described conventional testing methods present a number of problems. One of these arises from the fact that typical feeder cables have a loss of about 3 dB. Consequently, even if the antenna were to be removed entirely, a VSWR measurement carried out at the BS-end of the feeder cable would not detect a return loss exceeding 6 dB. The measured return loss, then, is as much a function of the actual loss of the feeder cable as it is a function of the antenna VSWR. The measurement accuracy can be improved by measuring/assessing the feeder cable loss and compensating for this loss in the measurement system. However, even if the loss of the feeder cable is known, the accuracy of the measurement will still be very poor. Thus, despite the fact that an accurate measurement of transmitted power can be made at the BS-side of the feeder, the amount of this transmitted power that is actually transmitted through the antenna is uncertain because of the very limited accuracy of the VSWR measurement.
The statistical methods also have problems, despite their simplicity. To begin with, the accuracy of these methods is limited. Furthermore, statistical methods can only be used when communication traffic is exchanged. In the absence of such traffic, it is impossible to tell whether it is simply the case that no one is calling, or whether calls are not being received because the antenna has malfunctioned. This can be a serious problem for base stations that are situated in remote locations. For example, consider a base station, located on an island in an archipelago, that hasn't exchanged a call during the entire month of November. Because of the faraway location, this may simply be the natural result of no one having attempted to place a call. However, it is also possible that a storm in October broke the RX antenna. In this situation it is necessary for the operator to have a way of determining whether a repair person needs to be sent to the island.